Praise be to Allah.
If he or she had an excuse, such as sickness, menses, nifaas (post-partum bleeding) and the like, because of which he or she delayed making up those days or delayed fasting the six days, then there is no doubt that he or she will attain that particular reward, as was stated by the scholars.
But if he or she had no excuse; rather they simply delayed that fast until Dhu’l-Qa‘dah, then the apparent meaning of the text indicates that they will not attain the particular virtue [of fasting six days of Shawwaal], and that [this fast] is a Sunnah connected to a particular time, which has passed, just as if he missed fasting the first ten days of Dhu’l-Hijjah and the like, fasts for which the time has passed. This special virtue has now passed, but he may still attain the reward of fasting in general terms.
Shaykh ‘Abd ar-Rahmaan as-Sa‘di (may Allah have mercy on him)
Al-Fataawa as-Sa‘diyyah (p. 230)